This invention relates to an improved monostable multivibrator for eliminating the fluctuation of output pulse width related to changes in the repeated cycle of an input pulse signal used for trigger signal generation.
As shown in FIG. 1, a conventional monostable multivibrator consists of a gate circuit (for example, an OR circuit) 1, a differentiating circuit 4 comprised of a differentiating capacitor 5 and a differentiating resistor 6, a driver circuit or a transistor switching circuit 2 comprising a transistor 3 connected to the output of gate circuit 1 where circuit 2 drives differentiating circuit 4, and a voltage comparison circuit (for example, an inverter) 7 that uses the output of differentiating circuit 4 as an input and feeds back the output as one input to gate circuit 1. The operation is such that when a trigger pulse is impressed on the other input terminal of gate circuit 1, an output pulse is obtained from the output terminal OUT of voltage comparison circuit 7.
In order to operate the conventional monostable multivibrator shown in FIG. 1, a trigger pulse with a narrower pulse width than that of the output of the monostable multivibrator is necessary. This trigger pulse is obtained by shaping the input pulse signal shown in FIG. 2(a) that is supplied to the input terminal IN of the trigger pulse forming circuit T. In the case of forming a trigger pulse from a rising (or falling) input pulse signal by the above trigger pulse forming circuit T, the low level voltage of the trigger pulse when the trigger pulse is a positive pulse, or the high level voltage of the trigger pulse when it is a negative pulse, vibrates at the rising (or falling) of the input signal to the trigger pulse forming circuit. This is illustrated in FIG. 2(b) where a trigger pulse waveform is shown as obtained with trigger pulse forming circuit T where illustrative circuitry for effecting the function of circuit T is described in co-pending Application Ser. No. 859,539, filed Dec. 12, 1977 where the logic of the circuitry disclosed in the foregoing application would be reversed. It should be understood that circuit T may also be implemented by circuitry other than that of the foregoing co-pending application. When the monostable multivibrator is operated by the trigger pulse width of the monostable multivibrator output pulse shown in FIG. 2(c) fluctuates at the time when the repeated cycle input pulse signal of the trigger pulse forming circuit changes returning the monostable multivibrator from the metastable state to the stable state and that coincides with the vibrating portion of the above-mentioned trigger pulse reference levels where in this specification the reference levels are chosen, for purposes of illustration, to be the low level side of a positive pulse and the high level side of a negative pulse.
If such a monostable multivibrator is used, for example, in the pulse count wave detector of an FM receiver, it would be disadvantageous in that the demodulated signal from the pulse count wave detector would appear distorted.